Objectives This study aimed to explore the effects of different vermicompost application rates on the yield, quality of Citrus reticulata ‘Hongmeiren’ and soil health, determine the optimal application rate, and provide a theoretical basis for improving soil health and enhancing yield and quality of Citrus reticulata ‘Hongmeiren’.

Methods A two-year consecutive field experiment was conducted in Pinghe County, Fujian Province from 2022 to 2023, with three-year-old Citrus reticulata ‘Hongmeiren’ as the test material. Five treatments were set up in the experiment: chemical fertilizer only (CK), and on the basis of chemical fertilizer application, applying vermicompost at rates of 3.75 t/hm² (T1), 7.5 t/hm² (T2), 15 t/hm² (T3) and 30 t/hm² (T4), respectively. The same total amounts of nitrogen (N), phosphorus (P₂O₅), and potassium (K₂O) were applied across all five treatments. At the harvest stage, soil samples were collected to determine soil physicochemical properties and biological characteristics, and calculate the soil health index (SHI). The fruit yields were investigated, and the quality indices of fruit were measured. The economic benefits were computed. The entropy weight method (EWM)-TOPSIS (Technique for order preference by similarity to an ideal solution) and hierarchical cluster analysis were adopted to conduct a comprehensive evaluation of yield, quality and economic benefits of each treatment, so as to determine the optimal application rate of vermicompost.

Results Compared with CK, the application of vermicompost significantly increased soil organic matter, available K, exchangeable Mg, potassium permanganate-oxidizable carbon and protein content, as well as soil respiration rate in both 2022 and 2023, while significantly reducing soil bulk density. No significant effects were observed on soil available Mn, Fe, Cu and Zn. Both linear and nonlinear soil health evaluation results indicated that the SHI in the 0−20 cm soil depth under vermicompost treatments was significantly higher than that under CK. In 2023, the SHI was positively correlated with fruit yield and the solidity-acid ratio (P<0.01). In 2022, there was no significant difference in the fruit yield between the vermicompost treatments and CK, but the transverse and longitudinal diameters of fruits were significantly increased by the application of vermicompost. In 2023, the fruit yields of vermicompost treatments were significantly higher than that of CK, with T2 treatment achieving the highest yield of 38.47 t/hm². The vermicompost treatments not only significantly increased the transverse and longitudinal diameters of fruits, but also enhanced the content of soluble solids and the solidity-acid ratio, thus improving both the external and internal fruit quality. In both 2022 and 2023, vermicompost treatments increased the total output value and net income, with the economic benefits in 2023 being generally higher than those in 2022. The T2 treatment obtained the highest net income of 17.46×10⁴yuan/hm² in 2023, representing an increase of 102% compared with CK. The EWM-TOPSIS comprehensive evaluation showed that the score ranking of each treatment was T3>T4>T2>T1>CK in 2022, and T2>T3>T4>CK>T1 in 2023. Hierarchical cluster analysis results revealed that T2 and T3 treatments were clustered into one group. Overall, the effects of vermicompost application on soil properties, as well as the yield, quality and economic benefits in 2023 were superior to those in 2022.

Conclusions The application of vermicompost can significantly improve the soil physicochemical properties, and enhance soil health level, and increase the yield, quality, and economic benefits of ‘Hongmeiren’, with a better effect achieved by consecutive application. The application rate of vermicompost 7.5−15 t/hm² is recommended for 3-year-old ‘Hongmeiren’ citrus under the experimental condition, further verification is required for orchards with other tree ages and soil types.